This disclosure relates to improvements to the handover process that takes place when a mobile communications device (user terminal) is required to cease communicating with a core network through one base station and begin communication through another base station. The most common reasons for such handovers to be required are because either the user terminal or the base station detects deterioration of the signal quality on the wireless communications link between them. This can be because the mobile device is moving out of range of the base station, but other changes in the wireless environment, such as changes in congestion or interference levels, may also make a handover appropriate. Handover may also occur when a user “roaming” on a network other than his “home network” (the one to which he subscribes) moves into range of a base station of his home network: in such a case a handover to the home network is desirable as soon as signal quality between the user terminal and the home network meets a predetermined threshold, regardless of the signal quality on the other network, because this will allow the user to use any facilities specific to his “home” network, and avoid paying the higher charges usually required for connection through a network other than the user's home network.
Unless the context requires otherwise, the term “base station” should be interpreted in this specification to mean any device or apparatus with which a terminal may communicate wirelessly in order to allow the terminal to communication with a backhaul connection to a core communications network. It includes, for example, access points (wireless routers) for “WiFi” (IEEE 802.11 standard) access networks, as well as the base stations used in cellular telephony.
The decision to initiate a handover, and the selection of a new link to which to hand over, is typically based on signal strength—thus, of the base stations signaling availability (having capacity to accept a connection and authorized to make connection with the mobile unit) a connection is established with the base station generating the strongest signal.
Handover can be between two cellular base stations, and in such cases is usually relatively straightforward when both base stations operate according to the same protocol. In many cases both stations are controlled by the same base site controller, which can coordinate the process. In a cellular system with permanent base stations, it is conventional to maintain a “neighbor list” for each base station which can be used to inform the mobile unit of the base stations to which handover is most likely to be possible.
However, increasingly, handsets are capable of operation in two or more different radio access technologies, for example cellular (UMTS (3G) and its packet data protocol GPRS) and WiFi (IEEE 802.11), and it is possible that a handover may be required between two base stations operating on different radio access technologies. One particular scenario occurs when a user device which has been working to an indoor short-range base station, operating on WiFi protocols, is to continue the session after leaving the premises, and therefore the range of the base station. Even within the cellular network, difficulties can arise because of the ongoing process of upgrading mobile networks from 2G (2nd generation) to 3G to 4G, which can result in neighboring base stations having different capabilities. For example some base stations may be not be capable of supporting both circuit-switched and packet-switched traffic.
In existing systems, loss, or deterioration, of contact with the currently-serving base station leads to an attempt by the mobile unit to seek a strong signal from another base station to which the mobile unit has access rights, and to arrange handover to that base station from the currently-serving base station. However, by relying simply on signal strength, a loss of service quality may arise because of a deterioration in other properties. For example, for a voice call (for example using VoIP—Voice over Internet Protocol) or VoLTE (Voice over Long Term Evolution Protocol), low latency and jitter are important characteristics, whereas for gaming or video streaming applications, high bit rate is a priority.
It is known, for example from patent specifications US2003/069018 (Matta), US2007/026861 (Kuhn) and WO2011/033173 (Valtion), to provide a ranking of neighbor base stations in terms of “quality of service” parameters, to allow the user to select a suitable candidate for handover. This requires the user to be familiar with the capabilities required for the applications running on the handset at the time, including any background applications.